Two major causes for oscillation in a control system are the presence of hysteresis and incorrect setting of the control parameters with respect to the process being controlled. The latter may be eliminated using advanced control techniques, such as with Neural Network-based controllers, or other controller types providing adaptive control for non-linear processes. The hysteresis is due to the friction in the mechanical elements of the control system such as valves, actuators, slack in associated linkages, and gear backlash, as well as some electrical characteristics of some components (for example the inductance of a solenoid). This oscillation may not be eliminated by presently available control techniques. Hysteresis manifests itself by a temporary lack of response whenever the mechanical device, for example, an actuator, changes direction. The temporary lack of response is due to the frictional or other forces in the system which must be overcome before movement occurs. The force needed to overcome the friction or other forces, once movement begins, is translated to momentum. This momentum causes the measured process output to exceed the setpoint or desired process output, requiring a second correction signal to recompensate. This correction signal will again lead to another momentum component, requiring a reverse correction. Repeated corrections are thus required, even during periods in which the setpoint remains constant. The oscillation causes premature wear of mechanical parts by causing unnecessary repetitive motion. The motion is also wasteful because energy must be expended by the system to maintain the oscillations.
In the HVAC marketplace, hysteresis is mostly related to mechanical factors and it is more critical with pneumatic control due to the added friction in the control devices. Electronic controllers and electronic actuators reduce the inherent hysteresis in the system, but cannot eliminate it entirely. Thus while in some systems hysteresis may be reduced by choice of control components, it may not be totally eliminated. Furthermore, in some applications, use of components that exhibit large hysteresis may be required to satisfy specific application requirements.
Since there are no known theoretical solutions to overcome hysteresis, the prior art response to hysteresis involves recognizing the occurrence of hysteresis in a system, and then fixing the control signals. This is done using a deadband control assistant which takes over control of the process from a normal control device (e.g. opens the control loop) when oscillation is recognized. In the prior art deadband control methods, a region is predefined around the setpoint in which oscillation is likely; this region is defined as the deadband range, having an upper and a lower deadband limit. Whenever the monitored process condition or process output, for example, temperature, enters the deadband range, the error of the system is set to zero. Subsequent calculations by the control system of the control signal are performed as if the error were zero.
While this solution can eliminate oscillation during quasi-steady-state conditions (hereinafter simply quasi-steady-state), it can result in a significant offset from the actual setpoint by as much as one-half the deadband range. The larger this offset, the greater the chance that a change in setpoint may not be registered by the control system. (For example, if the process output lies equidistant from the old and new setpoints.)
The large steady state offset occurs because the prior art deadband methods do not recognize when the control system is exhibiting a dynamic operation (which occurs during normal operation or under normal conditions when the process is approaching the setpoint) versus quasi-steady-state operation (when the error between desired and measured process output will not improve significantly). Enabling deadband control in a process exhibiting dynamic operation results in control perturbations and temporary instability. Prior art deadband control methods eliminate the dynamic operation-related instability by sacrificing system response time or sizing the deadband range large enough to contain setpoint overshoot.
Thus there is some need for a deadband control assistant which brings the process as close as possible to the setpoint without the offset previously caused by prior art deadband control methods. The resulting control system will have higher accuracy, since it reduces the wide range around the setpoint where the prior art deadband control assistant could not operate. At the same time, such a system would provide the oscillation control necessary to reduce mechanical part wear, and increase energy efficiency by eliminating the energy unnecessarily used to support oscillation around the setpoint.